1. Field of the Invention
The present invention relates to a cathode for electrolysis of an alkali metal chloride. More particularly, it relates to a cathode which requires a lower cell voltage in a process for producing an alkali metal hydroxide by an electrolysis of an aqueous solution of an alkali metal chloride by using a cation exchange membrane.
2. Description of the Prior Art
As a process for producing an alkali metal hydroxide by an electrolysis of an aqueous solution of an alkali metal chloride, a diaphragm method has been mainly employed instead of a mercury metod in view of a prevention of a public pollution.
It has been proposed to use an ion exchange membrane in place of asbestos as a diaphragm to produce an alkali metal hydroxide by electrolyzing an aqueous solution of an alkali metal chloride so as to obtain an alkali metal hydroxide having high purity and high concentration.
On the other hand, it has been proposed to save energy in the world. From the viewpoint, it has been required to minimize a cell voltage in such technology.
Certain processes have been proposed as processes for lowering a cell voltage. Among these processes, as a relatively advantageous process, it has been proposed to reduce a cell voltage by using a porous gas permeable cathode by dividing a cathode compartment into two parts; filling a catholyte in the part partitioned by the cathode and a membrane; feeding an oxygen-containing gas such as air in a space between the cathode and a wall of the cell so as to diffuse the gas in the gas permeable cathode whereby hydroxyl group is rapidly formed by reacting water with oxygen.
Such cathode is prepared by incorporating silver or a platinum group metal as a catalyst so as to accelerate the formation of hydroxyl group and incorporating a water repellent material such as polytetrafluoroethylene and paraffin so as to prevent a leakage of the resulting alkali metal hydroxide or water.
In the conventional process a desired porous cathode has been prepared by mixing the catalytic component with an aqueous dispersion of polytetrafluoroethylene, etc. as the resin having such water repellency.
Polytetrafluoroethylene is used for said purpose imparting water repellency as well as maintaining configuration as the cathode by combining with the catalytic component. The resulting cathode, however, is gradually broken down to reduce the water repellency and the function as the cathode is deteriorated.
The inventors have studied the reason why such phenomenon is caused, and have found that it relates to the particle size and the melting point of polytetrafluoroethylene.
The polytetrafluoroethylene used in the conventional process is in a form of rough particles having an average diameter of 500.mu. and has a melting point of about 327.degree. C. When only such polytetrafluoroethylene resin is used, it has been difficult to prevent the deterioration of the function of the cathode even though the process for preparing the cathode is improved.